1. Field of the Invention
The present invention relates to the extraction of oil from oil shale and tar sands.
2. Description of the Prior Art
Oil shales and tar sands represent two major sources of oil which, to date, have not fully been exploited, primarily due to the previously low cost and adequate supply of liquid crude oil and process difficulties in separating oil from oil shale and tar sand.
Nonetheless, the estimated reserves of oil existing in oil shale and tar sands throughout the world is immense and if a simple, efficient process for extracting oil from oil shale and tar sands could be developed, it would benefit the art.
Oil shales are typically fine-grained rocks resulting from the consolidation of mud, clay or silt, typically containing on the order of 20 to 50 gallons/ton of an organic, oil yielding material termed kerogen. Large oil shale deposits are found in the United States in Colorado, Utah, Wyoming and Texas. Kerogen is wax-like in nature, is characterized by low solubility in hydrocarbon solvents and typically will not flow unless heated to above 400.degree. F.
Tar sands, on the other hand, typically comprise sand, clay and silt saturated with a heavy, viscous bitumen which will typically be on the order of 5 to 30 percent by weight of the composition. Tar sand formations are highly cohesive and have a sticky, molasses-like consistency in warm weather.
Numerous processes have been proposed by the art to extract oil from oil shale and tar sands; in general, these have typically involved retorting at high temperatures or solvent extraction procedures, usually at high temperatures.
Typical of the high temperature retorting-type procedures are those disclosed in U.S. Pat. Nos. 2,601,257 (contact with a heavy shale oil at 700.degree.-800.degree. F.), 2,881,126 (contact with a hot oil bath and multiple step evaporation), 3,117,072 (retorting at high temperature and pressure with a high hydrogen concentration), 3,281,349 (retorting--cat cracking--at high temperature), 4,155,832 (hydrogenation utilizing a Ziegler catalyst in an alkyl benzene or light oil fraction solvent) and 4,161,441 (retorting and cracking oil shale).
Representative of typical solvent extraction processes conducted at elevated temperatures and typically elevated pressures include those described in U.s. Pat. Nos. 2,596,793 (methylene chloride), 3,929,193 (combination of certain solvents plus S in the 0 oxidation state), 4,130,074 (incomplete extraction using a solvent vapor/solvent system--typically a halogenated hydrocarbon and water) and 4,166,022 (super heated steam).
Defensive Publication based on U.S. Ser. No. 700,489, Long et al. (861 O.G. 703), discloses the extraction of oil shales with, e.g., mono- or di-methyl amine, at super critical conditions.
Recently a number of extraction processes have been suggested which are urged to be "low temperature" extraction processes. For example, such are disclosed in U.S. Pat. Nos. 3,941,679 (halogenated hydrocarbons), 4,029,568 (aromatic aliphatic and halogenated hydrocarbons), 4,046,668 (halogenated hydrocarbons), 4,046,669 (halogenated hydrocarbons), 4,055,480 (halogenated hydrocarbons), 4,057,485 (halogenated hydrocarbons), 4,057,486 (suggesting a plurality of aliphatic, aromatic and halogenated hydrocarbons may be used, but involving a critical water concentration) and 4,160,718 (specific to tar sands, involving a water slurry and a hydrocarbon oil).
In addition to the above processes which can relatively easily be classified as "retorting" or "solvent extraction" techniques, certain hybrid processes or esoteric processes have also been described, for example, in references such as the following U.S. Pat. Nos.: 3,074,887 (CO.sub.2 at high temperature and pressure), 3,346,481 (powder stream involving vaporizing components), 3,448,794 (in situ, super heating the oil shale), 4,108,760 (extractant gas, including amines), 4,156,463 (in situ, with steam and an amine), 3,497,005 (sonic energy), 4,135,579 (alternating current electric field) and 4,153,533 (microwaves).
However, all of the above processes, through such apparently offering one or more benefits to the art, are subject to one or more faults, for example:
For "retorting" type operation, high temperatures are required, necessitating high energy use, typically without good heat recovery. For a classical retorting process, quite often substantial amounts of residue result, lowering process yields.
Further, in thermal recovery systems such as in situ or surface retorting, the high temperature causes decomposition of oil and loss of recovery and undesirable fractions are generated that act as a contaminant in the recovered oil. Also with in situ retorting, charring of the oil to carbon is difficult to avoid.
For solvent extraction at high temperature, the energy input to the process is high, and thus this process is subject to the same faults as retorting processes. Further, at high temperature unless extremely efficient solvent recovery equipment is utilized, solvent losses can be high.
A fault fairly common to a large number of prior art processes, at least insofar as oil shale is concerned, is that the oil shale must be finely comminuted, which can lead to additional costs prior to active processing.
For hybrid systems, the complexity of the processing, of course, leads to substantial cost increases, an important economic disadvantage.
While many of the low temperature extraction processes seemingly overcome some of the above benefits, one substantial problem encountered is that typically the solvent utilized is either a complex mixture or a halogenated hydrocarbon, which results in relatively low yields at surprisingly high solvent costs.
A further disadvantage of many of the above prior art processes is that they are specific to processing oil shales or tar sands, and are not of universal application.